A group of Tory grandees and former ­senior diplomats will tomorrow launch a devastating attack on David Cameron's flagship Eurosceptic policies, warning that they pose a threat to British influence in the European Union.

On the eve of the European elections, the Tory leader stands accused of adopting a "rigid commitment to impotence" after he pledged to withdraw from the main centre-right grouping in the European parliament.

Cameron, who will appear alongside highly conservative EU allies in Warsaw tomorrow, goes into the European elections next Thursday on the most hardline ­Eurosceptic ticket of any mainstream political leader since Britain entered the EEC in 1973.

Cameron also says that a future Conservative government would be prepared to break with convention by reopening the Lisbon treaty, which is designed to streamline the working of the EU after its recent expansion.

The fears of Britain's most senior serving diplomats, one of whom described the Tory plans as "bonkers", are only being voiced in private.

But the Guardian has spoken to four Tory grandees, as well as two of Britain's most senior retired diplomats, who voice those fears in public. Lord Patten, the mastermind of the Tories' 1992 ­election victory, and former home secretary Lord Brittan both criticise Cameron's tactics, with Patten describing them as "unwise".

Lord Kerr of Kinlochard, a former head of the Foreign Office who was Britain's ambassador to the EU at the time of the Maastricht treaty negotiations in 1991, is also highly critical.

"I do not understand a rigid commitment to impotence," he said. "I do not understand why [the Czech and Polish parties who will form a new group with the Tories] are preferable to Angela Merkel or Nicolas Sarkozy, or why they think the route to influence lies that way."

Lord Wright of Richmond, head of the Foreign Office in Margaret Thatcher's final years as prime minister, questioned Cameron's decision to try to reopen the Lisbon treaty. "It will be a formidably difficult negotiation," he said. "There will be very few allies."

Lord Tugendhat, a European commissioner between 1977 and 1985, said it would be a "great tragedy" if the Tories tried to renegotiate a ratified Lisbon treaty once the party is in office.

Retired diplomats are careful about speaking in public. However, the strength of their language reflects Foreign Office concern that Cameron will trigger the worst crisis yet in Britain's relations with the EU. Their fears centre around Cameron's two key policies:

• The Tories have pledged to withdraw from the EPP-ED grouping, which consists of all the major centre-right parties within the EU. Instead, Cameron will establish a new group, principally with Poland's deeply conservative Law and Justice party, whose leaders have banned gay rights marches. It will also include the Czech ODS party, whose founder, Václav Klaus, disputes that global warming is man-made. Cameron met leaders of the ODS in the Czech Republic last night and in Warsaw tomorrow will meet leaders of the Law and Justice party.

The Tories have also been talking to the Lavtian Fatherland and Freedom party, some of whose members attend ceremonies to commemorate a Latvian unit of the Waffen SS.

• A future Conservative government will also offer a referendum on the Lisbon treaty if it has not been ratified by all member states. In the more likely scenario that it has been ratified, Cameron says he "would not let matters rest". Kerr was scathing of this stance.

He said: "Everyone is fed up with institutional treaties. The Tories owe it to us to tell us what they mean, because they will have to tell the world at the end of the first European council they attend, when they discover there is no majority for calling the intergovernmental conference to change the treaty as they propose."

His views are echoed by Tory grandees. Lord Patten, the former Hong Kong governor who went on to become a European commissioner, is dismissive of Cameron's pledge on the European parliament. "It is an unwise decision and will reduce the Conservatives' influence in the European parliament," he said.

Lord Brittan of Spennithorne, a former Tory home secretary who went on to become European trade commissioner, said: "I think it was a mistake for the Conservatives to leave the EPP. It will be a great loss. There is no doubt that the attempt to leave the EPP has annoyed a lot of the European leaders who are members of the EPP and are in government. It will make it more difficult to establish relations with them."

William Hague, the shadow foreign secretary, said: "It is not true that the Conservative party would retreat to the margins of Europe. Centre-right governments … are not only willing to work with us, they are already working with us behind the scenes in anticipation of the demise of this dreadful Labour government."